U668AA UNLOCK

Is this a phone you've had and used on AT&T in the past?  Or a phone you just bought used?

There are certain conditions that must be met before AT&T will unlock it.

Is it paid off?  Is the account it was used on current on its status, in other words... no outstanding or late bills?

Has it been used for the minimum amount of time AT&T requires. Post-paid and pre-paid accounts differ in the amount of time that a phone is required to actually be used on AT&T. 

If you think the phone meets those requirements, go to...

https://www.att.com/deviceunlock/

Try using the IMEI number instead of a phone number.

If the phone does not meet any of those requirements, then it ain't gonna get unlocked.

That phone was sold for AT&T prepaid. It had to be used on AT&T prepaid for 6 months in order to be eligible for unlocking.